The present application relates to fine particles of core-shell structure, each composed of a metal core particle and a shell layer, and also to a functional device incorporated therewith.
Among fuel cells are PEFC (polymer electrolyte fuel cell) and DMFC (direct methanol fuel cell) capable of converting chemical energy into electric energy through electrochemical oxidation of fuels such as hydrogen and methanol with oxygen or air. They are attracting attention because of their high energy efficiency and low environmental loads.
The fuel cell usually includes a plurality of unit cells placed one over another, each cell being constructed of a membrane-electrode assembly and separators holding it between them. Each separator has a gas passage for fuel gas or oxidizing gas. The unit cell is composed of an anode (fuel electrode or negative electrode), a cathode (oxidizer electrode or positive electrode), and a proton-conductive polymer electrolyte membrane held between them.
The electrode of the fuel cell is incorporated with a catalyst in the form of particles of noble metal of platinum group, such as platinum (Pt). The catalyst of noble metal accounts for a large portion of the production cost of fuel cells because noble metal such as platinum is an expensive material. This is a hindrance to the cost reduction and wide diffusion of fuel cells. To cope with this situation, it is technically important to contrive a new catalyst which needs less or no noble metal. To this end, extensive research and development are under way.
There are several ways of reducing the amount of platinum for the catalyst: one by reducing the size and increasing the surface area of platinum particles, one by incorporating platinum with other metals, one by alloying platinum with other metals, and one by replacing the platinum catalyst by a non-platinum catalyst.
One of solutions proposed so far to the foregoing technical problem is a catalyst in the form of fine particles of core-shell structure. Each particle includes a core particle and a shell layer of platinum atoms covering it. This idea stems from the fact that platinum atoms contributing to the catalytic action are those exposing themselves from the outermost surface of the catalyst particles and not those existing inside the catalyst particles. (See Patent Documents 1 to 5.)
Japanese Patent Laid-open No. 2002-231257 (paragraph 0006) (hereinafter referred to as Patent Document 1), which is entitled “Electrode catalyst for fuel cells and method for production thereof,” describes as follows.
“The electrode catalyst according to the application includes ruthenium particles whose surface is partly covered with a platinum layer. This structure reduces the amount of the platinum which exists inside the catalyst particles and does not contribute to reactions, and it also permits platinum atoms involved in reactions to be held on the surface of the particles. The electrode catalyst is particularly effective as the anode catalyst. Incidentally, the ruthenium particles should be left partly uncovered because they cannot oxidize carbon monoxide when entirely covered with platinum.”
Japanese Patent Laid-open No. 2008-34216 (paragraphs 0007 to 0013, paragraph 0029, FIG. 5) (hereinafter referred to as Patent Document 2), which is entitled “Electrode catalyst for fuel cell,” describes as follows.
“The electrode catalyst for fuel cell according to the first embodiment of the application is composed of a conductive carrier and a metal catalyst supported thereon, wherein the metal catalyst has surface parts each including 300 to 1300 platinum atoms joining together side by side to form a flat surface.”
Patent Document 2, which is entitled “Electrode catalyst for fuel cell,” describes as follows.
“The electrode catalyst for fuel cell according to the third embodiment of the application is the same one as defined in the first or second embodiment of the application which is characterized in that the surface of the metal catalyst has only the (111) plane.”
Patent Document 2, which is entitled “Electrode catalyst for fuel cell,” describes as follows.
“The electrode catalyst for fuel cell according to the fourth embodiment of the application is composed of a conductive carrier and a metal catalyst supported thereon, wherein the metal catalyst has surface parts each including 140 to 4000 platinum atoms joining together side by side to form a surface of spherical cap.”
Patent Document 2, which is entitled “Electrode catalyst for fuel cell,” describes as follows.
“The electrode catalyst for fuel cell according to the fifth embodiment of the application is the same one as defined in the first embodiment of the application which is characterized in that the metal catalyst has the center for the surface of spherical cap composed of the atoms of at least one species of Pd, Rh, Os, Ru, Ir, and transition metals which join together side by side and is composed of more than 500 atoms in total.”
Patent Document 2, which is entitled “Electrode catalyst for fuel cell,” describes as follows.
“The electrode catalyst for fuel cell according to the sixth embodiment of the application is the same one as defined in the fourth or fifth embodiment of the application which is characterized in that the surface of the metal catalyst is dominated by the (111) plane than the (100) plane.”
Patent Document 2 describes the electrode catalyst for fuel cell pertaining to the embodiment thereof with reference to FIG. 5 attached thereto, as quoted below.
“The electrode catalyst 20 for fuel cell is composed of the conductive carrier 21 of carbonaceous material and the metal catalyst 22 supported thereon. The conductive carrier 21 takes on a spherical shape. The metal catalyst 22 is composed of 140 to 4000 platinum atoms joining together side by side to form a monoatomic layer as the outer surface of the spherical cap 22a. The metal catalyst also has, inside the outer surface 22a, the inner surface of spherical cap composed of atoms of at least one species selected from Pd, Rh, Os, Ru, Ir, and transition metal (such as Cr, Mn, Fe, Co, Ni, Cu, and Zn). The spherical cap has the center 22b. The metal catalyst 22 is composed of more than 500 atoms and the outer surface 22a thereof is dominated by the (111) plane than the (100) plane.”
JP-T-2009-519374 (paragraph 0008, paragraph 0013) (hereinafter referred to as Patent Document 3), which is entitled “Nanoparticles of core-shell structure and method for production thereof,” describes as follows.
The application provides nanoparticles of core-shell structure, each including (a) a nanoparticle core and (b) a shell of crystalline substance formed thereon, the nanoparticle core being formed from a substance selected from metals belonging to Groups 3 to 15 of the periodic table, metalloid, lanthanide metals, actinide metals, and alloys and semiconductor compounds composed of at least two species of these elements. Any nanoparticle of metal or semiconductor to be used as the core is regarded as equivalent to that defined in the application disclosed in Patent Document 3 so long as it has the crystalline structure for epitaxial growth to form the shell.
JP-T-2010-501345 (paragraphs 0022 to 0031) (hereinafter referred to as Patent Document 4), which is entitled “Catalyst particles of core-shell structure, with the core being made of metal or ceramic, and method for production thereof,” describes as follows.
The catalyst particles according to the application disclosed in Patent Document 4 have the core-shell structure which exhibits the characteristic surface properties of noble metal (preferably platinum) in the form of polycrystalline bulk. Therefore, the catalyst particles of core-shell structure is characterized in that the shell is large enough for noble metal (such as platinum) constituting it in the form of polycrystalline bulk to exhibit its characteristic properties and the core does not contain the noble metal that constitutes the shell.
Each catalyst particle should have an average diameter (dcore+shell) ranging from 20 to 100 nm, preferably 20 to 50 nm, and more preferably 20 to 40 nm.
Each catalyst particle should have a shell thickness (tshell) ranging from about 1 to 20 nm, preferably about 1 to 10 nm, more preferably about 1 to 8 nm, and most desirably 1 to 3 nm. In addition, the shell of the particle should have a platinum layer with a thickness of at least three atoms. Any shell formed from platinum alloy should contain platinum atoms in combination with the atoms of the alloying element such that all the atoms constitute the shell layer with a thickness of at least three atoms. Any platinum layer thinner than specified above, especially a monoatomic platinum layer, does not contribute to specific activity.
Japanese Patent Laid-open No. 2010-92725 (paragraph 0011, paragraph 0013, paragraph 0023) (hereinafter referred to as Patent Document 5), which is entitled “Catalyst for fuel cell and method for production thereof, carbon particles carrying catalyst for fuel cell, and membrane-electrode assembly and fuel cell,” describes as follows.
The application disclosed in Patent Document 5 is concerned with a catalyst of core-shell structure for fuel cell, with the core being a fine particle of gold or alloy thereof and the shell being formed from platinum or alloy thereof. The foregoing catalyst for fuel cell can be applied to the catalyst layer of the positive electrode of the membrane-electrode assembly constituting the fuel cell. It imparts good durability to the fuel cell.
In order that the foregoing effect is produced, the shell layer of platinum or alloy thereof should have a thickness smaller than 2 nm. Such a small thickness permits the strain in the interface between the gold core and the platinum shell to propagate to the surface layer of the shell. The result is that the action to raise the ionizing potential due to tensile stress easily develops on the surface of the platinum shell layer. In addition, the shell layer may be made as thin as a monoatomic layer of platinum. On the other hand, the gold core should preferably be entirely covered with the platinum shell layer because platinum atoms coexisting with gold atoms in the surface of the nanoparticle catalyst adversely affect catalyst durability.
Japanese Patent Laid-open No. Hei 9-316504 (paragraphs 0009 to 0010) (hereinafter referred to as Patent Document 6), which is entitled “Aluminum superfine particles,” describes aluminum superfine particles, each being an aluminum multiply twinned particle in the form of pentagonal decahedron.
Japanese Patent Laid-open No. 2008-208418 (paragraphs 0007 to 0009) (hereinafter referred to as Patent Document 7), which is entitled “Molybdenum or tungsten particles or membrane composed of the particles and method for production thereof,” describes molybdenum or tungsten particles which have the crystal structure of face-centered cubic lattice (fcc) and which are thermodynamically stable or quasi-stable particles with a large particle diameter.
In addition, various researches on metal superfine particles are under way. For example, S. Iijima et al., “Structure Instability of Ultrafine Particles of Metals,” Phys. Rev. Lett., 56, 616-619 (1986) (FIG. 1 to FIG. 3) (hereinafter referred to as Non-Patent Document 1) reports the dynamic behavior of metal superfine particles observed under a high-resolution electron microscope, and O. Kitakami et al., “Size effect on the crystal phase of cobalt fine particles,” Phys. Rev. B, 56, 13849-13854 (1997) (Experiment, Results and Discussion) (hereinafter referred to as Non-Patent Document 2) reports the effect of particle size on the crystal phase of cobalt superfine particles.